Medium infrared transmittance and reflectance spectra of Bi_12GeO_20, Bi_12SiO_20, and Bi_12TiO_20 single crystals

Abstract

Room-temperature infrared spectra of Bi12GeO20, Bi12SiO20, and Bi12TiO20 single crystals have been obtained in the energy range 2000–400 cm−1 (5–25 μm). In the transmission region from 2000 to 1000 cm−1 (5–10 μm), the band parameters have been determined through a best fit by Lorentz functions. For the analysis of the reflectance spectra in the range 1000–400 cm−1 (10–25 μm), instead of using the Kramers–Kronig relations, we preferred to use classical oscillator method, which gives the best agreement with the experimental data. The determination of the oscillator parameters allowed us to deduce theoretical values for both the complex refractive index and the frequencies of TO and LO modes; the values for frequencies were in good agreement with those determined by other authors. The computational procedures are also presented and are critically discussed.